1. Field of the Invention
The present invention relates to a liquid crystal display device including a dispersed-type liquid crystal layer in which liquid crystal molecules are randomly distributed so that an incident light, impinging on the liquid crystal molecules, scatters.
2. Description of the Related Art
Liquid crystal display devices are used in a variety of technical fields, for example, in office automation devices such as personal computers in which a display is directly viewed, or projection type display devices in which an image is projected onto a screen.
A liquid crystal display device, in which a display is directly viewed, generally comprises a liquid crystal panel including a liquid crystal material inserted between a pair of opposite transparent plates, a polarizer and an analyzer arranged on either side of the liquid crystal panel, and a backlight for supplying a light toward the polarizer. Light emitting from the backlight selectively passes through the polarizer, the liquid crystal panel, and the analyzer, so that an observer can see the light passing through the analyzer as a display image.
A conventional backlight includes a light source lamp, and a light scattering layer arranged between the lamp and the polarizer so that a scattered light passes through the liquid crystal panel. Therefore, the scattered light includes light components travelling in many directions and passing through the liquid crystal panel, so that an observer can see the image on the display surface from many directions, such as the direction normal to the display and the directions oblique to the normal of the display.
A projection type liquid crystal display device is disclosed, for example, in Japanese Unexamined Patent Publication No. 3-152523, which comprises a liquid crystal panel, a polarizer, an analyzer, and a light source. Light emitted from the light source is made incident to the liquid crystal panel via the polarizer, and light emitted from the liquid crystal panel and passing through the analyzer is projected onto a screen by a projection lens. In this publication, a polarized light inducing means such as a beam splitter is arranged between the light source and the polarizer. The projection type liquid crystal display device must necessarily include the light source for projecting an image to a screen, and the light source of the projection type liquid crystal display device fundamentally differs from a backlight of a direct viewing type liquid crystal display device. Also, a light scattering layer cannot exist between the light source and the screen in a projection type liquid crystal display device, because an image on the screen will be blurred if a light scattering layer exists between the light source and the screen.
Liquid crystal display devices often use a twisted nematic liquid crystal. Recently, the use of other liquid crystal, such as a polymer dispersed-type liquid crystal is proposed (for example, refer to WO-A-8802128).
In the twisted nematic liquid crystal, liquid crystal molecules are aligned in parallel to the transparent plates in a predetermined direction, and liquid crystal molecules rise (tilt) perpendicular to the transparent plates when the voltage is applied to the liquid crystal. In the polymer dispersed-type liquid crystal, liquid crystal capsules are dispersed in a polymer, and liquid crystal molecules in the capsules are randomly distributed so that an incident light impinging on the liquid crystal molecules scatters. Japanese Unexamined Patent Publication No. 4-338923 discloses a modified example of the polymer dispersed-type liquid crystal, in which a liquid crystal is used in place of the polymer. That is, liquid crystal capsules are dispersed in a liquid crystal, and liquid crystal molecules in the capsules are randomly distributed.
In the liquid crystal display devices using a twisted nematic liquid crystal and of the type in which a display is directly viewed, such as displays in personal computers, the display may be clearly viewed when viewed from a certain direction and the display may be not clearly viewed when viewed from another direction. This is caused from the fact that liquid crystal molecules in the twisted nematic liquid are aligned in a constant direction and generally recognized as a visual angular characteristics of the twisted nematic liquid crystal display devices.
On contrast, in the polymer dispersed-type liquid crystal display devices, liquid crystal molecules in the capsules are randomly distributed and the incident light impinging against the liquid crystal molecules scatters, i.e., refracts and reflects in all the directions, and accordingly a specific visual angular direction, which may deteriorate quality of the display in the twisted nematic liquid crystal display devices, does not exist in th polymer dispersed-type liquid crystal display devices. Accordingly, regarding the displays of the type in which the display is directly viewed, it is preferable to use polymer dispersed-type liquid crystal display devices having a light scattering property.
In the use of polymer dispersed-type liquid crystal display devices, a white display spot is produced by the scattering of the impinging incident light when the voltage is not applied and a black display spot is produced by causing the liquid crystal molecules to rise relative to the transparent plates when the voltage is applied. However, there is a problem that, upon producing a white display spot by the scattering of the light, a portion of the light which can be utilized for producing a white display spot is decreased because components of the scattering light travel in many directions. That is, only a portion of the scattering light within a certain angular range that travels toward the analyzer and emits from the display zone can be utilized, and a portion of the scattering light that travels toward the outside of the display zone and a portion of the scattering light that returns toward the polarizer are not utilized.
Also, the inventors have found that, in the case of the polymer dispersed-type liquid crystal display device used in combination with the conventional backlight, the backlight itself includes the light scattering layer, and the incident light is subjected to the scattering effect twice, i.e., at the light scattering layer of the backlight and at the light scattering liquid crystal layer, so that a portion of the light which can be utilized is decreased. Therefore, the polymer dispersed-type liquid crystal display devices do not suffer from the problem of viewing angle characteristics but the problem of a relatively dark white display spot.